The present invention relates to a device for use in sensing pressure of a source of fluid or differential pressure between two sources of fluid.
The known devices for sensing pressures provide an arrangement such that two insulators are arranged opposite one another, with a sensing diaphragm mounted therebetween, and metal foils are attached to surfaces of the insulators. The surfaces are directed towards the sensing diaphragm to detect changes in the electrical capacitance between the sensing diaphragm and the metal foils produced when different pressures are exerted on the opposite sides of the sensing diaphragm.
FIG. 1 shows a known pressure sensing device of the type generally described above, including an overpressure protection function. The device is described in detail in Frick U.S. Pat. No.3,618,390, issued Nov. 9, 1971 corresponding to Japanese Patent Publication No. 23916/1974. The device essentially comprises a pressure sensing portion 10, a first cover portion 11, and a second cover portion 12. The first cover portion 11 and second cover portion 12 are secured to the pressure sensing portion 10 by means of screws or the like, not shown. The first cover portion 11 is provided with a first pressure chamber 13, into which a fluid at pressure P.sub.1 is inlet through a first introducing port 15. Also, the second cover portion 12 is provided with a second pressure chamber 14, into which a fluid at pressure P.sub.2 is introduced through a second pressure inlet port 16. A differential pressure between the pressures P.sub.1 and P.sub.2 is sensed by the pressure sensing portion 10.
This pressure sensing portion 10 essentially comprises a metal housing 17, a ring 31, and a stop ring 34. The metal housing 17 consists of metal portions 17A and 17B each formed with a cavity, which is filled with an insulating material 18, 19 such as glass or ceramics. The insulating materials 18 and 19 have their facing surfaces ground into dish shaped surfaces, to which metal foil 25, 26 are applied respectively. A sensing diaphragm 22 is arranged between the metal portions 17A and 17B, such that the sensing diaphragm 22 and the insulating material 18 form a first sensing chamber 20, and the sensing diaphragm 22 and the partially spherical surface of the insulating material 19 form a second sensing chamber 21. The sensing diaphragm 22 has its peripheral end portion welded to the metal portions 17A and 17B. The sensing diaphragm 22 is made of metal, and when the sensing diaphragm 22 is one capacitor plate, the metal foils 25 and 26 are other capacitor plates. Further, the metal portion 17A is provided with a first pressure receiving or isolation diaphragm 27, and the pressure P.sub.1 introduced into the first pressure chamber 13 acts on diaphragm 27. The metal portion 17A and the first pressure receiving diaphragm 27 form a first pressure receiving chamber 29, which communicates with the first sensing chamber 20 via openings formed within a ceramic tube 23. Likewise, the metal portion 17B is provided with a second pressure receiving or isolation diaphragm 28, and the pressure P.sub.2 introduced into the second pressure chamber 14 acts on the second pressure receiving diaphragm 28. The metal portion 17B and the second pressure receiving diaphragm 28 form a second pressure receiving chamber 30, which communicates with the second sensing chamber 21 via openings formed within a ceramic tube 24. The first sensing chamber 20 and first pressure receiving chamber 29, and the second sensing chamber 21 and second pressure receiving chamber 30 are filled with non-compressive filler liquid such as silicone oil or the like, the ceramic tube 23 and 24 each serving as a liquid flow passage for the filler liquid.
The metal member 17A has an annular ring 32 fastened thereto so as to surround the first pressure receiving diaphragm 27, whereas the metal member 17B has an annular ring 33 fastened thereto so as to surround the second pressure receiving diaphragm 28. The annular ring 33 is welded to a large ring 31 which receives housing 17. A stop ring 34, which encircles the housing 17 and is fitted in a cavity provided in the ring 31, is retained in position by means of screws 35 which extend through holes formed in a shoulder of the ring 31.
Thus, the pressure P.sub.1 introduced into the first pressure chamber 13 acts on the first pressure receiving diaphragm 27, and the pressure P.sub.2 introduced into the second pressure chamber 14 acts on the second pressure receiving diaphragm 28 to deflect the sensing diaphragm 22 in response to the difference between the pressures P.sub.1 and P.sub.2, whereby deflection of the sensing diaphragm 22 will cause changes in electrical capacity between the sensing diaphragm 22 and the metal foils 25 and 26 as acting the capacitor plates. The changes in electrical capacity may be taken out through lead wires which as connected to metal foils 25 and 26 through housing 17 and insulators 18, 19 and which pass through conduit 36 to thereby sense differential pressure between pressures P.sub.1 and P.sub.2.
In the pressure sensing device constructed as above, volumes of the first and second pressure receiving chambers are selected so that the sensing diaphragm 22 will bottom against the metal foil 25 or 26 in response to overpressure at diaphragm 27 or 28 before the latter will bottom against the corresponding metal member 17A or 17B. This insures that the overpressure stop will be positive, and because the deposited capacitor plates 25, 26 are very stable due to the massive amount of glass fused into the housing 17, there is no shift in calibration.
The prior art devices, however, do exhibit certain drawbacks.
Firstly, there are changes in the span of sensing differential pressures due to static pressure. That is, the outer peripheral surface of the housing 17 is at atmospheric pressure, whereas the interior of the housing 17 is under very large static pressure (for example, 100 kg/cm.sup.2). This large static force tends to inflate the housing such that the interior sensing chambers become somewhat enlarged. The proportion, at which the interior of the housing 17 becomes larger, depends upon the magnitude of high static pressure acting on the first sensing chamber 20 and second sensing chamber 21. This enlargement of the housing 17 means that the sensing diaphragm 22 is tensioned radially so that it becomes hardened according to the tension. As a result, the diaphragm 22 responds differently to a given P.sub.1 -P.sub.2 pressure differential than it would respond if it were under a different radial tension. This, of course, causes a change in electrical capacity between the sensing diaphragm and the metal foils 25, 26. For example, if in a first case the differential pressure .DELTA.P is 1 kg/cm.sup.2, the pressure P.sub.1 is 49 kg/cm.sup.2 and the pressure P.sub.2 is 50 kg/cm.sup.2 and in a second case the differential pressure .DELTA.P is still 1 kg/cm.sup.2, but the pressure P.sub.1 is 99 kg/cm.sup.2 and the pressure P.sub.2 is 100 kg/cm.sup.2, the greatly increased static pressure in the second case results in greater tension and hardening of diaphragm 22 with a resultant smaller deflection, despite the fact the .DELTA.P is the same in both cases. Thus, the change in electrical capacity in the second case is smaller than that of the first case. For this reason, the pressure sensing device poses a problem such that the output signal span (i.e. change in electrical capacity) changes with the magnitude of static pressure acting on the first sensing chamber 20 and second sensing chamber 21.
Secondly, there is an occurrence of overpressure error. As previously mentioned, the overpressure may be prevented in the pressure sensing device by the provision of an arrangement such that the sensing diaphragm 22 impinges upon the metal foils 25 and 26 before the pressure receiving diaphragms 27 and 28 impinge upon the housing 17. However, if the sensing diaphragm 22 is brought into close contact with either of the ceramic tubes 23 and 24 by the action of overpressures, a part of the sensing diaphragm 22 facing the bores in the ceramic tubes 23 and 24 is forced into the bores. The greater the overpressures, the greater the part of the sensing diaphragm 22 which will be forced into the bores of the ceramic tubes 23 and 24. This causes a problem such that when the overpressures have been removed, the deflection of the diaphragm is not completely returned to its original state due to a residual stress. As a consequence, if the sensing operation should be continued in a condition where the diaphragm is deflected, it leads to an error in the output signal.